Electromechanical overspeed limit switch



E. B. CANFIELD 2,938,091

ELECTROMECHANICAL OVERSPEED LIMIT SWITCH FilGd Oct. 2, 1957 INVENTOR.EUGENE B-CANFIELD RTTCWNEY United States Patent 2,938,091ELECTROMECHANICAL OVERSPEED LIMIT SWITCH Eugene B. Canfield, Pittsfield,Mass., assignor to General Electric Company, a corporation of New YorkFiled Oct. 2, 1957, Ser. No. 687,694 12 Claims. (Cl. 200-87) The presentinvention relates to an electromechanical limit switch and moreparticularly to an electromechanical overspeed limit switch operative ona small amount of magnetic flux and unaffected by forces due to gravity,linear acceleration, and the like.

It is general knowledge, that a large number of present day speed limitswitches are, in most cases, complex arrangements of springs and leversclosing a switch through the medium of centrifugal force and, usually,require slip rings to transmit the information from a rotating elementinherent therein. In addition to the complexity of structural detailsthese switches have a somewhat dubious reliability under some conditionsof operation. For example, a centrifugal overspeed switch is affectedby; forces due to gravity and linear acceleration, which decreases itsoperational efficiency.

The present invention provides an electromechanical overspeed limitswitch which reduces the number of parts usually associated with presentlimit switches, eliminates the necessity of a commonly used slip ringassembly and reduces the cost of manufacture and maintenance whilepermitting considerable increase in reliability of operation. Basically,the present invention discloses a switch closure which is a function ofarmature reaction so that a simple and reliable switch closure oropening is obtained as a function of speed. Hence, the present inventioncomprises a switch which operates on a small amount of magnetic flux,and when the magnetic intensity becomes sufliciently great, a pair ofmetallic switch leads become oppositely magnetized and the two leadsattract each other to complete the circuit. It will be obvious, ofcourse, that a switch can be constructed, in accordance with theprinciples of the present invention, so that the switch can be open ifdesired as a function of the speed.

An object of the present invention is the provision of anelectromechanical limit switch which is operative as a function ofspeed.

Another object is to limit switch with a switch ture reaction.

A further object of the invention is the provision of anelectromechanical overspeed limit switch operative as a function ofarmature reaction which controls the switch on a small amount ofmagnetic flux.

The attendant advantages of this invention will be readily appreciatedas the same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings in which like reference numerals designate like partsthroughout the figures thereof and wherein:

Figure 1 is a plan view of a preferred embodiment of the invention;

Figure 2 is a sectional view of the type of switch utilized in thepreferred embodiment of Figure 1; and

Figure 3 is a sectional view of a modification of the preferredembodiment of Figure 1, wherein the switch provide an electromechanicalclosure as a function of armais normally closed and opens when thedesired speed is obtained.

Referring now to the drawings, there is illustrated a preferredembodiment 10 illustrating a basic form of a limit switch, comprising apair of permanent magnet field poles P and P connected to a soft ironfield yoke 12, by any conventional securing means, and constructed in amanner similar to field poles used in a conventional DC. motor.Approximately ninety electrical degrees from the main field poles, P andP and approximately ninety mechanical degrees in the case of a two polemachine, are mounted cross field pole P and a switch 14, to be disclosedhereinafter, suitably provided with a pickup shoe P and withconventional type leads T and T A rotor 16 is concentrically providedwith respect to the field yoke 12, the main and cross field poles Pthrough P and the pickup shoe P The rotor may be a conventional squirrelcage rotor similar to that used in induction motors and frequentlyprovided with aluminum conductors integrally mounted into the slotsprovided therein. It is understood, of course, that the structuraldimensions of the various elements utilized in the structure embodyingthe present invention are a function of the specific electrical andmagnetic values required to perform a predetermined switching task for aspecific installation.

Figure 2 shows the salient details of the switch 14 which comprises aglass envelope 18 internally provided with a number of substantiallyparallel contact leaves 20 and 22 supported therein in such manner thatthe respective ends of the leaves protrude from opposite ends of theglass envelope. The leaves 20 and 2 2 are made of magnetic material andof suitable dimension to have sufiicient resiliency to perform theirpredetermined func tion. The two parallel leaves are supported withinthe glass envelope 18 so that their respective interior ends areoverlapped and spaced a predetermined amount, depending on theparticular installation and use intended. Switches, such as switch 14,are commercially available for use in the present invention. Therefore,the particular or specific construction of these switches do not formpart of the present invention; however, the application of theseswitches and the particular method of combining the switches with theother structure of the present invention, is considered the subjectmatter of the invention.

A magnet 24 is illustrated in Figure 2, in conjunction with the switch18 merely to illustrate the normal operation of the switch. Hence, whenthe magnet 24 is brought into proximity of the switch 18, and one leafis magnetized by one pole of the magnet and the other leaf magnetized bythe other pole, the two parallel and resilient leaves attract eachother. When the magnetic intensity becomes sufiiciently great, theleaves spring together to complete an electric circuit through theswitch 18 between the illustrated points A and B. in this manner, theswitch is simply a switch which operates on a small and predeterminableamount of magnetic flux.

In the operation of the present invention, the main field poles, P and Pcause flux to flow through the rotor 16. If the rotor is spun on itsaxis, an electromotive force (E.M.F.) will be generated across theconductors, not shown, under the main field poles. Assuming a squirrelcage rotor, all the conductors are of the main field flux will causecurrent to flow. The voltage generated is proportional to the speed ofrotation of the rotor 16, and accordingly, the rotor current 1s alsoproportional to speed.

Now, the rotor current causes armature reaction or cross field flux inan axis degrees from the main field flux associated with the main fieldpoles P and P This cross field fiux is also proportional to rotor speedand is picked up by the cross field magnetic circuit P and PConsequently, as the rotor speed increases, the cross field flux throughthe switch 14 increases, and when the cross field flux has increased toa predetermined value, the switch closes by the leaves 20 and 22 beingattracted to each other. i

If desired or necessary, the switch 14 can be made to close at anypredetermined specific speed by a number of different methods, and inparticular by any of the following enumerated means:

(a) By adjusting the cross field airgap by shimming the cross field poleP toward or away from the rotor 16;

(b) By adjusting the cross field airgap by moving the pickup shoe Ptoward or away from the rotor 16;

By shimming the main field pole P and P together or individually towardor away from the rotor 16, assuming, of course, that the poles areeither permanent magnets or else receive a fixed number of ampere turnsin field coils;

(d) By providing a trimmer winding wound around the main field poles Pand P such that flux from this winding will add to or subtract from themain field excitation;

(e) If the main field does not have permanent magnets, but is instead ofa wound construction, the main field current may be adjusted;

(f) By cutting a slot in one of the poles so that the reluctance of theparticular pole may be varied by positioning a soft iron wedge withinthe slot, since the reluctance would depend upon the amount of the slotfilled by the wedge.

Generally, adjustment for attainment of a specific predetermined speedmay be obtained by changing the main field or cross field airgaps, orchanging the main field excitation itself. Also, a change in rotorresistance while maintaining the same number of conductors will changethe speed at which the switch 14 operates. If desired, the switch 14 maybe made to close at one speed for one direction of rotation of the rotor16, and at another speed for the opposite direction of rotation. Thismay be accomplished by providing a small excitation winding aroundeither the switch 14 or the cross field pole P In this manner, the smallflux from this coil will aid armature reaction flux in one direction,and thus permit closure of the switch 14 at a lower speed. It will beobvious that the coils flux will buck armature reaction for the oppositedirection of rotation of the rotor 16 and will thus require a higherspeed of rotation for closure of the switch 14.

Figure 3 illustrates a modification 30 of the preferred embodiment 10,wherein a number of switches 32 and 34, similar in principle to theswitch 14 of Figure 2, and possibly similar in the specific constructionof the integral contact leaves, is utilized in the device of Figure 1.For some particular applications of the subject invention, it may bedesirable to have the switches 32 and 34 closed in their normaloperative attitude, and opening only when a desired speed is obtained.Accordingly, Figure 3 shows the construction of a main field pole toobtain this specific desirable feature. Hence, in the preferredembodiment 10, the modification 30 is substituted for one of the mainfield poles P or P while the switch 14 and the pickup shoe P is replacedby a cross field pole similar in construction and filnction to P In themodification 30, the main field pole is formed with a number of poletips A and B provided with switches 32 and 34, respectively, biasedclosed by the main field flux. The switches 32 and 34 are provided withconventional terminals T -T and T T respectively. It is generally wellknown that armature reaction reduces the flux on one side of a mainfield pole and increases the flux on the other side of the main fieldpole. Thus, as the speed of the rotor 16 is increased, the armaturereaction increases and is picked up by the cross field poles P and thenew pole substituted for the pickup shoe P and the switch 14 inFigure 1. Hence, this speed increase causes the flux in one main poletip, A or B to decrease and finally, when the armature reactionincreases sufficiently, the switch in the pole tip being demagnetizedwill open. It will be obvious, that for the opposite direction ofrotation of the rotor 16, armature reaction will demagnetize the otherpole tip causing the switch associated therewith to open. As enumeratedabove, adjustment of the switches 32 or 34, individually or together, tooperate at a specific predetermined speed may be accomplished bychanging the various airgaps betweenjhe poles or the field strengths.

It is obviously an inherent feature of the present invention thatinstead of using the modification 30, which is somewhat detailed inconstruction, the same results may be obtained by using a three leaveswitch for switch 14 in the preferred embodiment of Figure 1. In thismannor, a normally closed circuit in lieu of a normally open circuit canbe built using a switch of such construction that three parallel leavesare utilized.

The leaves, would comprise two adjacent and parallel magnetic leaves,similar in construction as 20 and 22, and one non-magnetic leaf adjacentand parallel to one of the magnetic leaves, wherein the non-magneticleaf and the adjacent magnetic leaf are normally closed, to form theclosed circuit, while the adjacent magnetic leaves remain open. In thismanner, the circuit through the non-magnetic leaf can be opened wheneverthe cross magnetic field flux reaches a predetermined amount and causesthe two magnetic leaves to attract each other. Hence, the normallyclosed circuit is predeterminedly opened as a function of speed.

In general, the use of a direct current and, especially, permanentmagnetic field is most desirable, howeventhe device will also perform onalternating current fieldexcitation. Further, the subject invention maybe used to produce current limit in an uncompensated motor or'generatoror any motor or generator where the switch may be placed in the armaturereaction flux, since the basic principle of the device gives a switchclosure as a function of armature reaction. Also, the present inventionmay be utilized as a speed regulator wherein the switch, such as switch14 in the preferred embodiment 10 will close at high speed and open whenthe speed is reduced, or switches 32 and 34 in the modification 30wherein the switches open at high speed and close when the speed isreduced.

Briefly, the present invention presents an electromechanical speed limitswitch which is much simpler than conventional centrifugal type speedcutouts. Further, the preferred embodiment 10, with or without themodification 30 has fewer parts than its comparable conventional speedlimit switch, and, accordingly, is relatively inexpensive tomanufacture. The present device is highly reliable and has no slip ringsor aging problem associated therewith and is stable in adjustment, sinceadjusting screws are not nearly so likely to loosen as on centrifugaldevices, for no adjustments are present on rotating parts. Since theswitches 14 or 32 and 34, when properly biased, will operate on a changeof flux no larger than the earths magnetic field, the device is capableof operation at extremely low speeds, and certainly much lower speedsthan practical with conventional centrifugal devices. Finally, unlikecentrifugal overspeed switches, the present invention is completelyunaffected by forces due to gravity, linear acceleration, or otherpossible unwanted inputs.

It should be understood, of course, that the foregoing disclosurerelates to only preferred embodiments of the invention and that it isintended to cover all changes and modifications of the examples of theinvention herein chosen for the purposes of the disclosure, which do notconstitute departures from the spirit and scope of the aaaaoai inventionas set forth in the appended claims on the to?- lowing pages.

What is claimed is:

1. A limit switch having a pair of permanent magnetic field poles with amain field flux associated therewith, a soft iron field yoke supportingsaid poles in diametrically opposite concentric positions, a cross fieldpole secured to said yoke and located approximately between said mainfield poles, a switch diametrically opposed from said cross field poleand having a pickup shoe operatively associated therewith, said switchhaving a number of parallel and resilient leaves predeterminedlyresponsive to a cross field flux between said cross field pole and saidpickup shoe and, in turn, to the main field flux.

2. A limit switch having a number of main field poles, cross field meansoperatively associated with said main field poles, a soft iron yokeconcentrically supported with said field poles, a rotor concentric tosaidyoke causing an armature reaction in the axis of said cross fieldmeans proportional to rotor speed, and switch means operatively coupledto said cross field means and operatively responsive to said armaturereaction.

3. An electromechanical limit switch adapted for use with an electricalcircuit, comprising a number of permanent magnetic field poles having amain field flux, a field yoke operatively associated with said fieldpoles, cross field pole circumferentially spaced from said magneticfield poles and coupled to said yoke, switch means fixed to said yokediametrically opposite said cross field pole, pickup means coupled tosaid switch means, a rotor concentrically mounted relative to said fieldyoke and operative in said main field flux to produce a cross magneticfield flux, said pickup means cooperating with said cross field pole todirect said cross magnetic field flux to said switch means to controlthe circuit.

4. An electromechanical limit switch for an electrical circuit,comprising a field yoke, magnetic field poles suitably secured to saidyoke, a cross field pole secured to said yoke and circumferentiallyspaced from said magnetic field pole, a pickup shoe coupled to said yokediametrically opposite from said cross field pole, a switch secured tosaid pickup shoe and mounted on said field yoke, a rotor concentricallymounted with respect to said field yoke to generate a cross field fluxproportional to rotor speed, said cross field flux directed by saidcross field pole and said pickup shoe to said switch to close theelectrical circuit at a predetermined rotor speed.

5. The invention as claimed in claim 3 but further characterized by saidswitch having a pair of parallel magnetic contact leaves magneticallyaffected by said cross field flux and, in turn, responsive to the rotorspeed.

6. An electromechanical limit switch having a yoke member, a number ofmain field poles diametrically spaced and secured to said yoke member,one of said main field poles having a number of pole tips, switch meansassociated with each of said pole tips and responsive to a main fieldflux, a number of cross field poles secured to said yoke member andcircumferentially spaced from said main field poles, a rotor memberconcentric to said yoke member and generating an armature reactionacross said cross field poles differentially affecting the magnetic fluxacross said main pole tips in response to rotor speed to predeterminedlydemagnetize and open one of said switch means.

7. A limit switch comprising main field magnetic circuit means includingmeans for producing a main field flux, cross field magnetic circuitmeans operatively associated with said main field magnetic circuitmeans, rotor means rotatably supported with respect to both of saidmagnetic circuit means for producing an armature reaction effective tochange the flux associated with both of said magnetic circuit means, andswitch means operatively coupled to one of said magnetic circuit meansand operatively responsive to said change in flux produced therein bysaid armature reaction.

8. A limit switch according to claim 7 wherein said main field magneticcircuit means includes a pair of main field poles, said cross fieldmagnetic circuit means includes a pair or" cross field poles and one ofsaid field poles has said switch means operatively coupled thereto.

9. A limit switch according to claim 8 wherein one of said crossfieldpoles comprises a pickup shoe operatively coupled to said switch means.

10. A limit switch according to claim 8 wherein one of said main fieldpoles has a pair of pole tips, and said switch means comprises a pair ofswitches respectively associated with said pole tips.

11. A limit switch according to claim 10 wherein both switches of saidpair are biased to a normally closed position by said main field fluxand one of said switches of said pair is caused to open by change in theflux due to the armature reaction produced at a predetermined rotorspeed.

12. The method of actuating a speed limit switch operable on a smallamount of magnetic flux comprising the steps of producing a main fieldflux, producing an armature reaction in cooperation with said main fieldflux to create a flux change as a function of speed, and subjectingtheswitch to said change in flux produced by said armature reaction toprovide switch operation as a function of armature reaction and thus asa function of speed.

References Cited in the file of this patent UNITED STATES PATENTS1,870,032 Young et a1. Aug. 2, 1932 2,112,214 Tognola Mar. 22, 19382,187,369 Uehling Ian. 16, 1940 2,207,506 Cox July 9, 1940 2,406,021Little Aug. 20, 1946 2,487,052 Hastings Nov. 8, 1949 2,550,605 SchenckApr. 24, 1951 2,833,879 Naul May 6, 1958

